1. Field of the Invention
The present invention relates to a method and an apparatus for assignment of Walsh codes in a mobile (wireless) communication system, and more particularly to a method and an apparatus for assigning Walsh codes having different bit lengths to respective spreading codes of a plurality of communication channels.
2. Description of the Related Art
The second-generation communication system IS-95 is a technique of CDMA (Code Division Multiple Access). The IS-95 communication system is an official name, regulated by the ITU-R (International Telecommunication Union-Radio communication sector), of “cdmaOne” (trademark) that is a wireless or mobile communication system currently practiced in and outside Japan. The IS-95 communication system adopts orthogonal modulation of a Walsh sequence. Namely, in the IS-95 communication system, respective communication channels are distinguished by the mutual orthogonality of Walsh codes (i.e., having no correlations among Walsh codes), each Walsh code being assigned (allotted) to the spreading code of an individual communication channel. The communication channels are control channels (such as a pilot channel, a synch channel for synchronization capture, or a paging channel for sending paging information) and traffic channels.
Here, an individual Walsh code to be assigned to the communication channels has a fixed 64-bit length in the IS-95 communication system. Therefore a Walsh code is arbitrarily assigned to a communication channel as the IS-95 communication system never fails in assignment of a Walsh code because of Walsh codes that has been earlier assigned.
In the meantime, the third-generation communication system IS-2000 (called “CDMA2000” in Japan), as an alternative, handles traffic channels having much higher data rates than those of channels supported by the IS-95 communication system. The communication system IS-2000 that is going to put into practice assigns Walsh codes of a 128-bit length at the maximum to communication channels for “spreading rate 1”, and Walsh codes of a 256-bit length at the maximum for “spreading rate 3”. For example, a Walsh code having a smaller bit length should be assigned to a higher data rate.
In the IS-2000 communication system that handles communication channels having different data rates, Walsh codes having different bit lengths are assigned to the corresponding communication channels as the spreading codes. If signals to which the assigned Walsh codes having different bit lengths are multiplexed, some Walsh code become unable to be orthogonally separated because of the regularity of Walsh function.
FIG. 18 of the accompanying drawings illustrates the manner in which Walsh codes having different bit lengths become unable to be orthogonally separated because of the regularity of Walsh function. In FIG. 18, “Wx_y” represents a Walsh code whose number is x and which has a y-bit length, and the maximum and minimum bit lengths are 16 and 2, respectively. As depicted in a hatched portion of FIG. 18, when the Walsh code “W2_4” is assigned, two Walsh codes “W2_8” and “W6_8” which are derived from the Walsh code “W2_4” and four Walsh codes “W2_16”, “W1_16”, “W6_16”, and “W14_16” which are further derived from the two Walsh codes “W2_8” and “W6_8” become unable to be assigned due to the mutual correlations among the derived six Walsh codes and the original code “W2_4”. This means that the Walsh code “W2_4” can be assigned to a spreading code only when all the other Walsh codes which are derived from the Walsh code “W2_4” are not occupied (assigned).
For this reason, the IS-2000 communication system would encounter with the following problem. When Walsh codes are arbitrary assigned likewise the conventional communication system IS-95, the remaining idle Walsh codes tend to be in a circumstance where no Walsh code in a small bit length for a communication channel of high data rate remains despite of a lot of idle Walsh codes in large bit lengths for communication channel for low data rate remain. As a result, subsequent assigning of Walsh codes having small bit lengths would tend to become impossible.
For example, assuming that four Walsh codes “W0_16”, “W1_16”, “W2_16”, and “W3_16” are already assigned as shown in FIG. 19, it is possible to further assign twelve channels for Walsh codes of a 16-bit length or four channels for Walsh codes of an 8-bit length. Nonetheless it would be impossible to assign any Walsh code having a 4-bit length or a 2-bit length.